FI96644B - A volumetric phase holographic element and a method for forming it - Google Patents

Abstract

Volume phase holograms and other holographic elements which include microscopic areas of a material having an index of refraction different from that of the holographic fringes.

Description

96644

Volumetric phase holographic element and method of its formation! This invention relates to volumetric phase holographic elements having regions of microscopic material in which the refractive index is different from that of holographic lines. Novel holographic elements may have increased diffraction power and / or other desirable properties. The invention also relates to a method for forming these elements.

Volume phase holograms, i.e., refractive index modulated holograms, are well known and are made of a variety of photosensitive materials, especially materials in which holographic lines are formed by photopolymerization or crosslinking.

Dichromatic gelatin (DCG) has been widely used to generate volume phase holograms. However, the mechanism by which a holographic image is formed in DCG has not been elucidated and a few mechanisms-20 mechanisms have been proposed. One such suggestion is that "cracks" or "cavities" are formed between the line planes in the gelatin, and the resulting difference in the refractive index of the air (in the slits or cavities) and the gelatin causes increased coefficient modulation. [See. A. Graube, Holographic. 25 Optical Element Materials Research ", U.S. Air Force * Office of Scientific Research Technical Report 78-1626 (1978, available from the U.S. Defense Technical Information Center as DTIC Technical Report ADA062692, especially pages 95-113.] The presence of" cavities "is disputed. in a previous article [RK Curran and TA

* Shankoff, The Mechanism of Hologram Formation in Dichromated Gelatin, "Applied Optics 9 (July 1970) 1651-1657, especially p. 1655], which concludes that" slits "provide an interface between air and gelatin.

35 In a publication published in 1980 [B.J.

Chang, Dichromated Gelatin Holograms and Their Applications, Optical Engineering 19 (1980) 642-648, cf. in particular pp. 642-643], yet another mechanism is proposed, the "molecular chain spring" model. Another mechanism - the interaction between isopropanol and gelatin, which causes cracks when isopropanol is removed, is proposed by J.R. Magarinos and D.J. Coleman [Hologra-5 phic Mirrors, Optical Engineering 24 (1985) 769-780].

These researchers have found that one way to support the hypothesis of the presence of on-cavities (microcavities) would be to demonstrate a loss of holographic power by replacing the air in such cavities with a liquid with a refractive index of 10 the same as gelatin, but they disagreed they have such a result. However, it is clear that these researchers are studying mechanisms rather than ways to convert optical properties.

A particularly useful composition in photopolymers for forming volume phase holograms is described and claimed in U.S. Patent 4,588,664 (May 13, 1986).

Herbert L. Fielding and Richard T. Ingwall). These photopolymerizable compositions contain a dye-sensitizer such as methylene blue, branched polyethyleneimine as an initiator of polymerization, and a free radical mechanism polymerizable ethylenically unsaturated monomer, preferably, for example, acrylic monomer and lithium monomer. A method for stabilizing holograms formed from this photopolymerizable composition is described and claimed in U.S. Patent 4,535,041 (Herbert L. Fielding and Richard T. Ingwall, August 13, 1985).

The invention relates to a volumetric phase holographic element formed by exposing a holographically photopolymerizable element. The element according to the invention is characterized in that the microcavities between the strips are at least partially filled with a material other than air, said material having a different refractive index than the matrix.

The invention also relates to a method for forming a holographic element. The method is characterized in that the photosensitive element is holographically exposed to laser light, the exposed photosensitive element is generated to obtain a volumetric phase holographic element with microcavities between the lines, and the microcavities are at least partially filled with a material having a different refractive index. with a material, wherein the photosensitive element is a light-polymerizable element.

As is well known in the art of holography and defined, volume step holograms record information as modulation of the refractive index of the medium in which the registration is made. The polymerization of the monomer in the photopolymerizable film thus registers the laser holographic exposure as a pattern of "polymer streaks" or "layers". The lines are parallel to the substrate in the volume phase reflection hologram and perpendicular to the substrate in the volume phase transmission hologram. The strip-forming polymer has a different refractive index than the material between the stripes, and the resulting modulation of light as a function of the coefficient differences allows the hologram to be reconstructed.

Examination of the fine-grained field of the holograms formed using the photopolymerizable compositions described in the aforementioned U.S. Patent 4,588,664 by light microscopy and scanning electron microscopy has shown the presence of air-containing microcavities between the photopolymer lines; the microcavities were generally round in cross-section.

Although the microcavities appear isolated in electron microscopy images, they are interconnected. Alternating planes of solid and porous (coupled microcavities) material occur in layers spaced according to the appropriate Bragg condition. The diameter of the microcavities is generally on the order of 35 to about 50 to 80% of the distance between adjacent solid phases (lines) and may be, for example, on the order of about half the wavelength of light reflected by a holographic reflective optical element. These microcavities 4,96644 contain air and their density varies as a function of exposure power. It is believed that these micropores are at least in part the result of horizontal diffusion of the monomer in the film to the areas where polymerization occurs. The development and drying of the photopolymerized film causes the removal of water or other solvent from the monomer, and air fills the resulting microcavities. Because the air-filled microcavities scatter some light, some turbidity may occur in the holographic element, depending on the size of the microcavities.

It has now been found that it is possible to reproducibly and reversibly or permanently replace at least part of the air in the microcavities with some other material without significantly altering the distance between the holographic beverages. The added material preferably remains permanently in the holographic element. In this way, desirable changes in optical properties / holographic performance can be achieved, as the effect of small differences between the photopolymerized (line) regions and the added material or coefficients or optical densities on the optical properties of the combination is amplified. A material that replaces the air in the microcavities may, for convenience, be termed a "micro-on cavity material."

The air replacement material in the microcavities is introduced into the holographic element by impregnating the desired substance or its solution therein, for example by wetting. The added material may be a substance capable of further reacting, for example a polymerizable monomer or a crosslinkable compound, and such further reaction effectively incorporates said material permanently into the holographic element. The incorporation of the polymerizable monomer may be done simultaneously with the polymerization initiator, or the initiator 35 may be added first and then the polymerizable monomer, which is more preferred. The initiator and monomer should be selected so as to avoid the introduction or formation of any undesirable discoloration.

5,96644

Particularly useful monomers for obtaining a polymer with a lower refractive index in microcavities are fluorinated, especially perfluorinated, monomers. The polymerization can be carried out by any known method, for example by ultraviolet initiation, which is suitable for the monomer and is compatible with the matrix matrix material. In certain cases, a thin film of the resulting polymer may also be formed on the surface of the hologram, and such polymer film 10 may provide improved physical protection, e.g., resistance to moisture or other environmental factors that would otherwise adversely affect the stability or optical properties of the hologram. In addition, such a surface polymer film can be used to attach or bond a hologram to another material or hologram.

The microcavity material may be a dye or other light absorbing compound to provide light absorption at a predetermined wavelength or wavelength band. This embodiment is particularly useful in providing holographic suction filters, for example to protect the eyes from lasers, as the resulting optical density (transmission) is a combination of reflected and absorbed laser light. Select a light absorbing agent to be appropriately added, the optical density can be increased with respect to one or more wavelengths in question while maintaining substantially unchanged transmission at other wavelengths, allowing improved user protection without undue or excessive image degradation in visible light, for example.

The added material may reduce the degree of turbidity of the holographic element due to the fact that the refractive index difference between it and the "line polymer" is less than 35 the difference between the line polymer and air. Although in some cases a reduction in diffraction power may result, especially if the hologram is a reflection hologram, the benefit of reduced turbidity may more than compensate for the reduction in diffraction power. If the refractive index of the added material is close to the refractive index of the line polymer in some region of the spectrum, for example the visible region, but is different in another region, for example in the infrared region, turbidity can be virtually eliminated in the first region and at the same time strong reflectivity in another area.

The selection of materials suitable for filling the microcavities can be made by selecting materials with the desired refractive index, followed by conventional tests to examine the permeability of the hologram to the material to be tested. It will be appreciated that suitable combinations of microcavity material and hologram matrix may vary depending on changes in either component. In general, it is desirable to use a microcavity material having a refractive index 20 that differs by at least 0.1 unit from the refractive index of the hologram matrix; the larger the refractive index difference, the stronger the effects. By selecting micro-on-telomaterials and solvents that do not swell the matrix or swell it very little, the distance between the lines 25 can be kept substantially unchanged; on the other hand, the distance between the registered line planes can be increased by selecting microcavity materials and solvents that swell the matrix.

Since the average refractive index of the fil-30 containing filled microcavities is higher than that of the film containing the filled microcavities, the maximum reflection wavelength shifts in the reflection holograms for longer wavelengths, i.e., in the red direction with a decrease in diffraction power. This feature can be used to adjust the maximum reflection wavelength in narrowband holography filters, as ♦ 96644 7 it is possible to pre-adjust the peak wavelength in the range of about 50 to 100 nm depending on the refractive index of the material selected to fill the microcavities.

In the case of transmission holograms, the presence of microcavity material changes the diffraction peak angle.

Because the power of such gratings varies periodically with lattice intensity (which varies with exposure level), the microcavity material can either increase or decrease the diffraction power of hi-10 lan.

If the microcavity material is optically active, i.e., has a different refractive index for right and left rotationally polarized light, the resulting hologram will have different properties with respect to these two to 15 polarized light. For example, if the microcavity material in the reflection hologram has approximately the same refractive index as the matrix with respect to otherwise polarized light, this type of rotating polarized light would pass the hologram with little or no attenuation, while light polarized in the opposite direction would be strongly reflected. A transmission hologram containing such a microcavity material would bend these two differently rotationally-polarized light at different angles.

It will be appreciated that the physical size of the material to be incorporated into the microcavities must be small enough for the material to enter the microcavities, and that this size will vary with the size of the microcavities.

The microcavity material may be selected to improve the stability of the hologram against the effects of the environment, for example humidity and / or temperature, or to reduce or eliminate the adverse effects resulting from contact with adhesives or plasticizers. (The stabilization treatment with sequential treatment with zirconium salt and fatty acid described in the aforementioned U.S. Patent 4,535,041 does not fill the microbes in the cavity of 8,966,444, nor does the cx used in Example 3 of the aforementioned U.S. Patent 4,588,664. -syanoetyyliak--acrylate).

For convenience, the information disclosed in the aforementioned U.S. Patent Nos. 4,588,664 and 4,535,041 are incorporated herein by reference.

The following examples are provided to illustrate the invention and are not intended to be limiting.

Example 1 A transmission hologram was prepared using a photopolymerizable film of the type described in Example 7 of the aforementioned U.S. Patent 4,588,664 and following the general development and stabilization procedure described in said example using helium-15 neon laser light (633 nm). The final thickness of the film after dry treatment (excluding the substrate) was about 6 yum. Analysis of the transmission spectrum of the dry hologram with a visible spectrophotometer showed a spectral band width of about 170 nm and a maximum diffraction ratio of 83% at 565 nm. When trifluoroethanol (refractive index 1.29) was absorbed into the dry hologram, the width of the spectral band decreased to about 60 nm, the maximum diffraction ratio increased slightly to 87%, and the wavelength corresponding to the maximum ratio shifted to approximately 635 nm. Trifluoroethanol was washed off the hologram by applying isopropanol and acetone a few times and the hologram dried. The hologram was then impregnated with General Electric Silicone Oil SF 99 (refractive index 1.39); the diffraction ratio was measured to be about 58% and the wavelength corresponding to the maximum ratio was about 630 nm.

Example 2; A small amount of a solution of 1% benzoin ethyl ether in xylene was placed on the hologram prepared as described in Example 1 and the xylene was allowed to evaporate. A small amount of perfluoropolyether diacrylate 9 was then added dropwise to the holographic plate.

O

H 2 C = CH-CO 2 -CH 2 CF 2 O (CF 2 O) x (CF 2 CF 2 O) y CF 2 CH 2 OCCH = CH 2 (molecular weight about 2,100; refractive index 1.31) was allowed to spread over the area of the holographic image and then covered with Figure 5 on a glass plate. The monomer-saturated hologram was irradiated with ultraviolet light (405 nm) for 10 minutes, followed by irradiation with ultraviolet light having a wavelength of 365 nm for 10 minutes. The Pei glass was found to stick in place after the first UV-10 exposure. The diffraction ratio was found to be about 20% after the second UV exposure; originally it was 74%. The coverslip was removed and the hologram was boiled in water for about 1.5 hours, after which the ratio was found to be about 22%. The hologram was removed from its glass substrate 15 as an intact but somewhat brittle self-straight permanent film.

Example 3

A solution of diethoxyacetophenone was dissolved in

II

A mixture of CF 2 (CF 2) g CH 2 O-O-CH = CH 2 and the perfluorinated monomer used in Example 2 was impregnated into a transmission hologram prepared as described in Example 1 using helium neon laser irradiation at 2 15 mJ / cm 2. The hologram was then irradiated with ultravio-25 counter light for 15 minutes. The diffraction ratio of the treated hologram was found to be 70%; the diffraction ratio was originally 22%.

Example 4

The procedure described in Example 2 was repeated using a reflection hologram with a diffraction ratio of more than 99% at 490 nm and UV exposure for 20 minutes. The diffraction ratio of the resulting hologram containing polymerized perfluorinated polymer in the microcavities was found to be 80% at a wavelength of 585 nm. After the treated reflection hologram was heated at 150 ° C for about 15 hours, its diffraction ratio of 10,966,444 was found to be 87% at 540 nm.

Example 5

A holographic mirror (reflection hologram) in the center of 5 photopolymerizable layers on a glass substrate, prepared in the general manner described in Example 1, was treated with a solution of erythrosine in trifluoroethanol. After evaporation of the solvent, the photopolymerized layer was pink in color. The color cannot be removed by washing with isopropanol, methanol or mixtures of these alcohols with water. Optical density (transmission) was measured spectrophotometrically at a few incident angles. The stained holographic mirror region had an optical density peak over 5 in the wavelength range of 15,530 to 555 nm at incidence angles of 0 to 40 °. By comparison, at the perpendicular angle of incidence, the original holographic mirror had an optical density peak at 2.8 at 580 nm and a colored non-holographic region had an optical density peak at 3.5 at 5 540 nm. The original holographic mirror was still visible through the reflected light. Based on these measurements, it was concluded that erythrosine at least partially filled the microcavities.

It will be appreciated that other binders and polymers capable of providing volume step holograms may be used in place of that used in the example above, provided that suitable microcavity materials can be absorbed into the hologram.

It will be appreciated that the holograms of this invention may be incorporated into other optical structures using suitable adhesives, for example, optical. epoxy adhesives. In addition, holograms can be encapsulated or sealed only at their edges to improve environmental stability.

35 Although the present invention has been described in detail and with reference to specific embodiments thereof, the art; . »U.i a-iti i ns - j 11 96644 It will be apparent to those skilled in the art that various changes may be made therein without departing from the spirit and scope of the invention.

Claims (15)

1. Volume phasic holographic element formed by holographic exposure of a photopolymerizable element, characterized in that the micro-hollow spaces between the bands are at least partially filled with a material other than air, said material having a refractive index other than the material. 2. Volume phase holographic element according to claim 10, characterized in that said refractive index is lower than the refractive index of the material. 3. Volume phase holographic element, characterized in that said refractive index is greater than the refractive index of the material. 4. A volume phase holographic element according to claim 1, characterized in that the material contains a branched polyethyleneimine and a polymerized ethylenic monomer. 5. Volume phase holographic element according to claim 4, characterized in that the monomer is an acrylic acid. 6. Volume phase holographic element according to claim 1, characterized in that the material is hydrophobic. 7. Volume phase holographic element according to claim 2, characterized in that the lower refractive index material is a polymerized, fluorinated monomer. The volume phase holographic element according to claim 7, characterized in that the surface of the holographic element has a thin layer of said polymerized, fluorinated monomer. ψ Volume phasic holographic element according to Claim 1, characterized in that said material is a material which increases the absorption of light of a path length reflected by the holographic element. 10. A method of forming a holographic element, characterized in that a holographically illuminated laser-sensitive element is irradiated, produces the exposed, light-sensitive element, thereby obtaining a volume phase holographic element with micro-cavities between the bands, and at least partially fills the micro-well compartments 5 with a material having a refractive index other than the material material of the holographic element, the photosensitive element being a photopolymerizable element. 11. A process according to claim 10, characterized in that the photopolymerizable element contains a color sensitizer, a branched polyethyleneimine and a free-radical mechanism polymerizable, ethylenically unsaturated monomer. 12. A process according to claim 10, characterized in that a polymerisable monomer is subsequently added to the microelectrums, which is then polymerized, forming a material having a lower refractive index than the matrix. 13. Process according to claim 12, characterized in that the monomer is a fluorinated monomer. 14. A method according to claim 12, characterized in that polymerization of a thin layer of the monomer is applied to the surface of the holographic element. 15. Volume phase holographic element, characterized in that the micro-halves between the bands are at least partially filled with a polymer formed by inserting a monomer into the micro-halves and polymerizing the monomer in the micro-halves, the polymer having a different refractive index than the matrix. i «